A technique for preventing a reverse rotation at a time when a diesel engine starts is conventionally known (for example, Patent Literature 1 (PTL 1)). In a single-cylinder diesel engine, however, a reverse rotation may occur not only at a time of starting but also during operation. For example, in a case where a flywheel returns (rotates in a reverse direction) due to an inertial force while a diesel engine is operating and a fuel is injected timely at that time, the reverse rotation may continue.